Is this way of notating a vocal alternative clear? This is the full score. In the vocal score it may be appropriate to add a reduced size (65%) stave with the alternative, but I'd prefer not to do that in the full score. (Bass clef, crotchet = 126)

IMO, I would include the last regular note (f#) also, as double stemmed upp/down.
I am not familiar with the word "oppure" so I would write "ossia" (preferably not italic but regular roman).
By the way, what is the font and size you use for lyrics? It looks very good.

Oppure = ‘otherwise’ or even ‘or.’ I use it rather than ‘ossia’ out of habit, as it is familiar to opera singers from its use in the standard (Ricordi) editions of Verdi and other Italian opera. My habit of using it may not be very logical, and may be no more than nostalgia for the fact that the first full score I ever owned was of Verdi's Otello (a score subsequently lost in my travels).

I used italic so that it was very clearly different from the libretto.

The font size is 20 point, Finale Lyrics (a compact variant of Times, I think). Minion Pro might be a better font for this, but I'm not sure whether or not the point size would be the same. I like the words to be clearly legible in a full score, even with a quite small stave size. In a vocal score, with rather larger staves, it might still be appropriate.